1. Field of the Invention
The present invention relates to a polarizer controlling polarization of light in the deep ultraviolet region.
2. Description of the Related Art
Polarizers are roughly categorized into prism polarizers and film polarizers.
Prism polarizers take advantages of birefringence and Brewster's angle to have excellent polarization characteristics. The prism polarizers, however, are thick and require a large installation space. Moreover, they have a limited incident angle which allows only polarization of normally-incident light, i.e. no obliquely-incident light is polarized.
On the contrary, film polarizers can be formed as a thin device requiring a smaller installation space, and have a larger incident angle which allows polarization of obliquely-incident light. A film polarizer is formed, for example, by rolling in one direction a glass substrate which has conductive particles such as silver halide particles mixed therein, and forming the particles into an elongated shape. These elongated silver halide particles produce anisotropy of electrical conductivity that imparts polarization characteristics to the film polarizer.
A wire grid polarizer is another type of the well-known film polarizers. This polarizer includes a glass substrate on which metal wires, e.g., aluminum wires, are placed equally spaced, and is provided with anisotropy of electrical conductivity as in the above described film polarizer. Such wire grid polarizers are discussed in Japanese Patent Application Laid-Open No. 2008-90238, and Z. Y. Yang et al., “Broadband Nanowire-grid Polarizers In Ultraviolet-Visible-Near-Infrared Regions,” (OPTICS EXPRESS Vol. 15, No. 15, pp. 9510-9519). In the latter publication, a polarizer is described in which metal wires of gold, silver, chrome, and aluminum are formed to have a pitch of 80 nm for the wavelength range from ultraviolet to near infrared (300 nm to 5 μm). The publication also discusses another structure that includes a CaF2 substrate having a wire grid of aluminum thereon.
Conventionally, these prism and film polarizers have been discussed to control polarization of visible light, infrared light, and ultraviolet light.
Exposure equipment uses light having a wavelength shorter than that of ultraviolet light (300 nm to 380 nm) to form more fine patterns. For example, the wavelength may be KrF (248 nm), ArF (193 nm), F2 (157 nm), or Ar2 (126 nm), for example. A polarizer is used in a polarized-light illumination system or an interferometer for stage position measurement of the exposure equipment that uses light in the above wavelength range as exposure light.
A film polarizer with elongated silver halide particles requires a substrate of fluorite (CaF2) or fluorine-doped quartz glass (SiO2) that is also highly transparent to ultraviolet light. In such a case, the substrate cannot be rolled with silver halide particles mixed therein. As a result, no elongated silver halide particle is formed and anisotropy cannot be imparted.
In addition, even the wires of the metals described in the above “Broadband Nanowire-grid Polarizers In Ultraviolet-Visible-Near-Infrared Regions” cannot realize excellent polarization characteristics for the wavelength range of deep ultraviolet. The wire grid polarizers have been used so far for infrared light. However, these polarizers cannot have closely-spaced metal wires or a high aspect ratio for light in the deep ultraviolet wavelength range, resulting in poor polarization characteristics for the light.